Solar Radiation

Health Area > Dermatology

The earth is continuosly showered with solar radiation: the electromagnetic energy emitted by the sun. This energy is the source of all that we know and yet, like many other good things, too much can be harmful. Among the spectrum of radiation that the sun emits, Infrared, Visible and Ultraviolet radiation command most of our attention. Visible light is generally considered begnin and won't be discussed here.

Ultraviolet Radiation (UVR)

The solar spectrum is divided into various portions according to wavelenght. UVR covers from 200 nanometers (nm) to 400nm. UVR is, in turn, divided into UVC (200-290 nm), UVB (290-320 nm) and UVA (320-400 nm). UVA is further categorized as UVA I (340-400 nm) and UVA II (320-340 nm), also called long and short UVA respectively. UVR is generally eredited with most of biologically significant sequela of sun exposure.

UVC

UVC, also known as GERMICIDAL UV, is very toxic. As this implies, it is lethal to many microorganism as well as to most plant life. In addition, it is carcinogenic to humans.Fortunately, virtually all UVC is filtered out by the ozone layer.

UVB-UVA

UVB makes up about 18% of the solar UV spectrum (prior to attenuation by the earth's atmosphere) but only about 1% of the UVR that gets to the earth's surface because it is largely filtered out by the ozone layer. Despite its relatively meager presence however UVB is associated with much of the damage caused to humans by sun exposure. Traditionally, UVB was credited as being the sole cause of sunburn and various skin cancers. Although still considered a major cause of skin cancers, UVB is no longer though to be acting alone. It now seems likely that other wavelengths (UVA) will be found to be involved in tumor formation, and perhaps, in some cases, even as the primary agent.UVA makes up about 75% of the solar UV spectrum and about 99% of the earthly spectrum. This is because UVA is largely unaffected by the ozone layer: Much more abundant, UVA is also much less energetic than UVB and thus traditionally has been thought to be biologically less significant. UVA is certainly a causative factor in photo-aging and many believe that it will be shown to be a major cause of some forms of skin cancer. It is also the portion of the UV spectrum most often associated with photosensitivities resulting from drugs or disease. The exact UVA wavelenghts (the Actions Spectrum) responsible for the various phenomenon are not yet defined. For these reasons it is now considered mandatory that any product claiming sun protection attenuate both the short and long UVA.

Infrared radiation (IR)

In descending wavelenght order IR (>700 nm) precedes visible light (400-700 nm) which is followed by UVR (200-400 nm). We are most concerned with the near IR from about 700 nm to 1,800 nm. IR is currently believed to be only a minor player in skin damage. It is thought to potentiate the harm caused by UVR and can, almost by itself, cause at least one human skin disease, erythema abigne. Importantly however, it is the IR portion of the solar spectrum that is mostly responsible for the heating effects of the sun. As the producer of heat, Ir radiation is the cause of a lot of displeasure for us during the summer months and the source of a great deal of comfort for us during the winter. As we increase our ability to block the more immediate threat of UVR, we will no doubt turn our attentions to blocking the less important but bothersome IR.